Click the [New Floor] button and it will pop up a box telling you the cost of adding a new floor, as shown below.

Once you have confirmed, it will add a new floor in your retail store, providing you with a new set of 3×3 space for setting up new functional units, like the following screenshot shows.

You may expand your retail store to have a maximum of 3 floors.

Please note that the operating overhead of your retail store will increase in direct proportion to the number of floors it has. So you may want to only expand if you see sufficient consumer demands for your products.